Moulding compositions based on unsaturated polyester resins normally consist of resin, filler, catalyst for curing, internal mould release agents and reinforcing fibers. The original compositions were in the form of a putty-known as dough moulding compounds (DMC). Following the discovery that the incorporation of certain Group II metal oxides and hydroxides such as CaO, Ca(OH).sub.2, Mg(OH).sub.2 caused thickening of the resin system, compositions which were tack free known as bulk moulding compounds (BMC), sheet moulding compounds (SMC) and thick moulding compounds (TMC) became available. BMC was originally based on chopped glass fibers whilst SMC and TMC were based on chopped strand mat and roving cuttings respectively. In SMC the resin was applied to the chopped strand mat and, after compacting, was rolled up between cover sheets of, for example, polyethylene. After about 3 days the polyethylene could be readily peeled off to yield a tack free sheet, weighing typically from 3-8, preferably from 4-8, more preferably from 5-6 Kg/m.sup.2, which could be loaded into a heated press and moulded at temperatures of 120.degree.-150.degree. C. under pressure. In TMC, which allows for three dimensional orientation of the fibers, the roving cuttings were ordered in parallel strands and fed together with resin paste material, for example, by a pump, into the nip of two contrarotating rollers. The compound was then passed through fast running take-off rollers onto a carrier film of, for example, nylon which, together with a cover film, was fed on to a conveyor belt. This resulted in a sheet weighing, typically, from 5-40, for example, from 6-20, or from 20-40 Kg/m.sup.2, which was cut, for example, by hand and laid often as a single piece, in a mould. A typical machine for producing TMC is disclosed in US-A-No. 3932980, and a comparison of SMC and TMC is made by M. Yamada et al, 33rd Annual Technical Conference, 1978 Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, Inc., Section 4-8 pages 1-6.
The thickening of resin by Group II metal oxides/hydroxides is due to a combination of the formation of covalent bonds and co-ordinate bonds. It is difficult to control. Ideally the impregnation mix should be low in viscosity to allow good impregnation of the fibers, should thicken rapidly after impregnation and should reach a maximum viscosity which does not change on storage. Such behaviour is represented by a graph of viscosity vs. time (days) marked "IDEAL" in the accompanying drawing. What actually happens when using such metal oxides/hydroxides is also shown in another graph of the accompanying drawing marked "TYPICAL SMC". The rate and extent of thickening depends on the resin used to such an extent that not only the normal resin parameters have to be controlled (i.e. acid value and viscosity) but also hydroxyl value and molecular weight distribution. In addition, since the metal oxide/hydroxide is influenced by the presence of moisture and carbon dioxide in the atmosphere, special storage precautions are required to prevent even further variations in maturation on storage.
It is also known to make other types of thermosetting resins, e.g. vinyl esters, into BMCs and SMCs, but it has previously been necessary to make special modifications to the resins to allow the thickening reaction with Group II oxides and hydroxides to occur, this being because the standard vinyl ester resins often have very low acid values.
All of the above-mentioned compositions were highly filled, but if unfilled systems were required the filler was omitted and a solvent based resin was pre-impregnated on to reinforcing fibres, the solvent removed and the resulting tacky prepreg rolled up between sheets of film. However, the sheets of film were difficult to remove from these prepegs due to the tackiness of the pre-impregnated fibre.
One method of overcoming the disadvantages with unfilled pre-impregnated reinforcements is described in GB-A-1319243 and GB-A-1318517. Examples are given in these patents of polyester resins which when blended with styrene monomer are solid and can be used to impregnate reinforcing fibres when molten.
In our EP-A-0083837, we disclosed the use of crystalline resins to thicken both filled and unfilled ethylenically unsaturated polymer moulding compositions based on standard resins thus eliminating the need for special resins made for moulding compositions. Because the thickening mechanism is a physical one a number of other advantages occur:
(i) no metal oxide/hydroxide is required and hence, in contrast to when a metal oxide is used, no special storage precautions are required to prevent further variations in maturing, PA0 (ii) indeed, no maturation period is required, the compositions being ready for moulding as soon as they have cooled and PA0 (iii) storage stability is much improved.
Whilst such compositions have proved useful in industry they have shown the disadvantages of reducing the effectiveness of low profile and/or low shrink additives when they are incorporated in the moulding compositions. In particular, they at least to some extent reduce gloss, allow the fibre glass pattern to become more visible, increase surface waviness and allow shrinkage to occur.